Chronic myeloid leukemia (CML) is one of the four major categories of leukemia. The other three are acute myeloid leukemia, acute lymphoblastic leukemia, and chronic lymphocytic leukemia.

Regardless of the type, all leukemias begin in the blood-forming cells in the bone marrow. Each type of leukemia gets its name from how fast cancer tends to grow (acute cancer grows fast; chronic grows slowly) and also the type of blood-forming cells from which the malignancy developed.

CML is a chronic leukemia, meaning it tends to grow and progress slowly. CML is also a myelogenous leukemia, meaning it starts in immature white blood cells known as myeloid cells.

Certain changes in DNA can cause normal bone marrow cells to become leukemia cells. People with CML generally have the Philadelphia chromosome, which contains the abnormal BCR-ABL gene. The BCR-ABL gene causes white blood cells to grow in an abnormal, uncontrolled way, causing leukemia.